The invention relates generally to the field of electrical power converters and inverters. More particularly, the invention relates to techniques for preventing or forestalling failure of motor drive circuitry due to overheating.
Power inverters and converters typically employ power modules to create a desired output current waveform, which is used to power various devices, such as motors and other equipment. The frequency and amplitude of the output current waveform may affect the operation of the device such as by changing the speed or torque of a motor, for example. Some power modules create the desired output current waveform through pulse width modulation, wherein power semiconductor switches such as insulated gate bipolar transistors (IGBTs) are caused to switch rapidly on and off in a particular sequence so as to create an approximately sinusoidal output current waveform. Furthermore, high transistor switching speeds tend to produce a smoother, more ideal sinusoidal waveform, which may be desirable in some applications. For example, in heating, ventilating, and air conditioning systems a smoother sinusoidal waveform will reduce system noise and vibrations.
Higher transistor switching speeds may tend, however, to increase the junction temperature of the transistors, which may result in more mechanical stress and increased rates of transistor failure over time. Attempts have been made to reduce transistor failure by limiting the maximum absolute transistor junction temperatures. However, these techniques have failed to account for the increased stresses that tend to occur under start-up conditions or low-speed conditions, wherein the transistors tend to experience high current at low output frequency. For example, a power converter used as a doubly fed induction generator converter may typically operate in a maximum power point tracking (MPPT) condition, at which the converters operate at a relatively low frequency and high output current. Such low frequency conditions may contribute to electrical failures of the converter.
It may be advantageous, therefore, to provide a system and method of reducing the thermal stresses due to a continuous MPPT operation mode of the converter. Specifically, it may be advantageous to provide a method of reducing temperature variations of the transistor junction, i.e. the semiconductor chip itself, and the case, i.e. the package in which the semiconductor chip is contained.